Underwater sound is exploited in both passive and active sonars for the detection, localization, and tracking of both quiet and radiating sources. Sound is also useful for obstacle avoidance, creation, of acoustic images of an underwater scene, and undersea communication. In many such instances, it is necessary to increase the response of an acoustic receiving system to sound arriving from a particular direction. Two methods to accomplish this include signal processing techniques (i.e., beamforming) and acoustic lens techniques. In both cases, an array of sound-sensitive elements is generally used in a receiving system. In the lens case, acoustical images of underwater scenes can be produced by focusing sound onto elements of an array, much like the human eye focuses light onto the retina. Acoustic lenses generally reduce the signal processing and hardware demands on a system which is particularly important for compact, real-time systems. Both techniques can also be used to directionally broadcast sound.
It is particularly challenging to generate high-resolution acoustical images of scenes at low frequencies on small, platforms because physical apertures (i.e., array sizes) must be many wavelengths long. Techniques such as towing long arrays or using platform translation (i.e., synthetic aperture) are commonly used in this regard.
In acoustic communication, similar compactness constraints on the receive end of a system make it difficult to reject multipath interference and ambient noise in favor of direct path symbols. Multipath interference, for example, is a factor that limits bit rate.